Support ValueFrom.Expression on Workflow level argument

# Summary

```yaml
apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
generateName: demo-
spec:
arguments:
parameters:
- name: var
valueFrom:
expression: "name-{{=sprig.randAlphaNum(5)}}"

```

What change needs making?

# Use Cases

When would you use this?

---

**Message from the maintainers**:

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The issue provides a Workflow YAML example using valueFrom.expression on a workflow-level parameter, but names no files, tests, or entry points. Trace how workflow-level arguments are evaluated, then verify that the expression resolves successfully and add coverage for the shown use case.

Written by the indexing model from the issue text.